Plasma CVD film-forming device

ABSTRACT

A plasma CVD film-forming device forms a film on a semiconductor substrate in such as way that the film quality and film thickness of a thin film becomes uniform. The plasma CVD film-forming device to form a thin film on a semiconductor substrate includes a vacuum chamber, a showerhead positioned within the vacuum chamber, and a susceptor positioned substantially in parallel to and facing the showerhead within the vacuum chamber and on which susceptor the object to be processed is loaded and the central part of the showerhead and/or the susceptor constitutes a concave surface electrode.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a device for forming a thin film on asemiconductor substrate by the vapor growth method using plasma, andparticularly it relates to a semiconductor processing device that ischaracterized by the shape of a showerhead and/or a susceptor.

[0003] 2. Description of the Related Art

[0004]FIG. 1 outlines a conventional parallel-flat-plate type plasma CVDfilm-forming device. The conventional plasma CVD film-forming devicecomprises a vacuum chamber 1, a showerhead 2 positioned upright withinthe vacuum chamber substantially horizontally and a susceptor 3positioned substantially in parallel and facing the showerhead withinthe vacuum chamber 1.

[0005] In the vacuum chamber 1, an exhaust port 5 leading to a vacuumpump (not shown) is used for vacuum exhausting the inside of thechamber.

[0006] At the base of the showerhead 2, multiple fine holes 11 foremitting a jet of material gas are positioned. The showerhead 2 is alsolinked to a material gas supply tank 6 through a line 10. On the line10, a mass flow controller 8 for controlling a flow of the material gasis positioned. An RF power source 4 is also electrically connected tothe showerhead 2, and functions as one side of the electrodes.

[0007] The susceptor 3 is normally an aluminum column within which aheater 14 is embedded. The susceptor 3 is supported by a support 12 andcan be also rotated, for example, by a rotating mechanism. The susceptor3 is also connected to ground 13 and functions as the other electrode.On the surface of the susceptor 3, a semiconductor substrate 9 is loadedand is fixed by vacuum fastening, etc.

[0008] Operation of the conventional plasma CVD film-forming device isexplained below.

[0009] First, gas within the chamber 1 is vacuum exhausted by the vacuumpump from the exhaust port 5, and preferably a low pressure ismaintained within the chamber 1.

[0010] Next, a preselected material gas flowing from the material gassupply tank 6 is controlled by the mass flow controller 8 at apreferable flow. A material gas controlled at a preferable flow istransported to the showerhead 2 through the line 10 and is jetted outfrom the multiple fine holes 11 provided at the base, toward thesemiconductor substrate.

[0011] After a flow is stabilized, a radio-frequency (RF) electric fieldis generated between the showerhead connected to the RF power source andthe susceptor 3 grounded to the earth 13. The above-mentioned materialgas within the chamber 1 is ionized and a plasma state occurs. Atoms ofthe ionized material gas show a chemical reaction at a reaction regionon the semiconductor substrate, and a desirable thin film is formed onthe semiconductor substrate.

[0012] As a material gas, silicon source gasses such as SiH₄,DM-DMOS[(CH₃)₂Si(OCH₃)₂] and TEOS, fluorine source gasses such as C₂F₆,oxidizing gasses such as oxygen and inert gasses such as Ar or He can beused.

[0013] The type and quality of a film formed on the surface of thesemiconductor substrate 9 change according to the type, flow andtemperature of the material gas, the RF frequency type, and the plasma'sspatial evenness.

SUMMARY OF THE INVENTION

[0014] The evenness of a film formed on the semiconductor substrate andthe evenness of plasma density at the reaction region are closelyrelated. As shown in FIG. 1, a distance between the susceptor 3 and theshowerhead 2, i.e., a distance between the semiconductor substrate 9 andthe showerhead 2, is fixed for a conventional plasma CVD film-formingdevice. In general, in the parallel-flat-plate type plasma CVDfilm-forming device, an electric field intensity distribution generatedbetween two plane electrodes (Ø250 mm) has the property of beingstrongest at the center and gradually weakening toward the outer edgealong a radius. In the film-forming region of a semiconductor substrateof Ø200 mm, the intensity distribution is approximately ±5%.Consequently, the electric field around the center of the semiconductorsubstrate 9 is relatively stronger than the electric field toward theouter edge along the radius, and the plasma density is also higher andthe reaction of a material gas becomes more active. As a result, a thinfilm formed becomes thicker at the center, and the film quality becomesuneven at the center and at the outer area of the center.

[0015] This problem conventionally has been dealt with by controllingthe flow or mixing ratio of gas supplied, the value of RF frequencyapplied and RF power energy. However, when these parameters are changed,the quality of the generated film and the film-forming speed change andstability of the process deteriorates. Particularly, if the mixing ratioand the flow of a material gas considerably affect the film quality,this problem becomes more serious.

[0016] It is important to resolve the problem of the evenness of a filmdue to the need for a larger diameter for semiconductor substrates inthe future.

[0017] Consequently, an object of this invention is to provide a plasmaCVD film-forming device that forms a thin film with an even film qualityand an even film thickness on a semiconductor substrate.

[0018] Other object of this invention is to provide a plasma CVDfilm-forming device that forms a thin film with an even film quality andthickness for a substrate with an diameter of more than 300 mm.

[0019] Another object of this invention is further to provide a plasmaCVD film-forming device at a low manufacturing cost and with a simpleconfiguration.

[0020] To accomplish the above-mentioned objects, a plasma CVDfilm-forming device according to this invention comprises the followingmeans:

[0021] A plasma CVD film-forming device for forming a thin film on asubstrate, comprises: (a) a vacuum chamber; (b) a showerhead positionedwithin said vacuum chamber; and (c) a susceptor positioned substantiallyin parallel to and facing said showerhead within said vacuum chamber andon which said substrate is loaded, wherein the showerhead and thesusceptor are used as electrodes and have surfaces facing each other, atleast one of which surfaces is concave.

[0022] In the above, in an embodiment, the concave surface is arotatably symmetrical surface around an axis of the showerhead or thesusceptor.

[0023] In another embodiment, a distance between said showerhead andsaid susceptor satisfies the following relation:

fd=|dc−da|/da×100

fd=1%˜100%

[0024] wherein:

[0025] fd is a deformation ratio of the central part of saidshowerhead's surface that faces said substrate,

[0026] da is the average distance between said showerhead and saidsusceptor at an outer perimeter position of said substrate,

[0027] dc is the average distance between said showerhead andsaid,susceptor at a point on a radius of a distance equivalent to dafrom the center of said substrate.

[0028] Further, in yet another embodiment, a distance between saidshowerhead and said susceptor satisfies the following relation:

fd′=|dc′−da′|/da′×100

fd′=1%˜100%

[0029] wherein:

[0030] fd′ is a deformation ratio of the central part of saidsusceptor's surface that faces said substrate,

[0031] da′ is the average distance between said showerhead and saidsusceptor at an outer perimeter position of said substrate,

[0032] dc′ is the average distance between said showerhead and saidsusceptor at a point on a radius of a distance equivalent to da′ fromthe center of said substrate.

[0033] In an embodiment, a distance between the showerhead and thesusceptor becomes greater toward the center and it becomes greatest atthe center.

[0034] In the above, deformation ratios fd and fd′ can range from1%˜100% independently or concurrently. In an embodiment, deformationratio fd or fd′ is 5-35%.

[0035] Deformation ratio fd or fd′ may be determined to rendersubstantially uniform a distribution of electric field intensity overthe substrate while forming a film thereon.

[0036] In the above, distance da or da′ may be in the range of 3 to 300mm, preferably 5 to 100 mm. Difference |dc−da| or |dc′−da′| may be inthe range of 0.3 to 50 mm, preferably 0.5 to 20 mm.

[0037] Deformation ratio fd or fd′ can be indicative of uniformity ofquality and thickness of a film formed on a substrate. Additionally, adistance, dw, between the susceptor and the substrate can be indicativeof quality and uniformity of thickness of a film and may be in the rangeof 0.1 to 10 mm, preferably 0.1 to 5 mm.

[0038] In an embodiment, the susceptor can have a diameter sufficient tosupport a substrate having a diameter of 300 mm or larger. A film can beformed on a large substrate.

[0039] Other conditions for processing a substrate can be the same asthose conventionally employed. The showerhead may supply a material gascontaining a compound selected from the group consisting of compoundswhich can be expressed by Si_(x)O_(y)C_(z)N₁H_(m) wherein x, y, z, 1,and m are independently zero or an integer, including SiH₄, Si(OC₂H₅)₄,(CH₃)₂Si(OCH₃)₂, and C₆H₆. To the material gas, an additive gas such asHe and O₂ may be added in an embodiment. An radiofrequency power may beapplied between the showerhead and the susceptor. Further, the susceptormay comprise a heater.

[0040] The present invention also relates to a method for forming a thinfilm on a substrate by using the aforesaid plasma CVD film-formingdevice. The method may comprise: (I) loading a substrate on thesusceptor; (ii) controlling the atmosphere in the vacuum chamber; (iii)applying energy between the showerhead and the susceptor; and (iv)forming a thin film on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIG. 1 roughly illustrates a conventional plasma CVD film-formingdevice.

[0042]FIG. 2 shows a first example of the plasma CVD film-forming devicethat possesses a showerhead according to this invention.

[0043]FIG. 3 shows variation examples of a showerhead according to thisinvention.

[0044]FIG. 4 shows a second example of the plasma CVD film-formingdevice according to this invention.

[0045]FIG. 5 shows a third example of the plasma CVD film-forming deviceaccording to this invention.

[0046]FIG. 6 is a graph showing the relation of the surface depth and adistance from the electrode center according to the difference in theshowerhead lower surface shape.

[0047]FIG. 7 is a graph showing the relation of the degree of concavityat the central part of the electrode and the film thickness of thesemiconductor substrate center.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0048] This invention is explained referring to the following figures:

[0049]FIG. 2 roughly illustrates the first example according to thisinvention. The same symbols are used for the same materials used inFIG. 1. The first example of a plasma CVD film-forming device forforming a thin film on an object to be processed according to thisinvention comprises a vacuum chamber 1, a showerhead 20 positionedwithin said vacuum chamber and a susceptor 3 positioned substantially inparallel to and facing said showerhead within said vacuum chamber and onwhich susceptor said object to be processed is loaded. This exampleshows a case where a distance between said showerhead and said susceptorbecomes greater toward the center and it becomes the greatest at thecenter.

[0050] The plasma CVD film-forming device shown in FIG. 2 operates Inthe same way as the conventional plasma CVD film-forming device shown inFIG. 1. However, in this invention, by transforming the surface shape ofelectrodes, distribution of an electric field within the surface isimproved and the evenness of a film to be formed is improved.

[0051] Preferably, a base 21 of the showerhead 20 comprises a concaverotating surface. Here, a rotating surface is defined as a curvedsurface that is generated by rotating a curved line on a plane around astraight line on the same plane.

[0052] In FIG. 2, a distance between the showerhead 20, i.e., an upperelectrode, and the semiconductor substrate 9 is the greatest at thecentral point 22 and gradually lessens toward the outer edge along aradius.

[0053] The deformation ratio fd of the center 24 of the upper electrode21 is defined as follows:

fd=|dc−da|/da×100

fd=1%˜100%

[0054] wherein:

[0055] fd is a deformation ratio of the central part 24 of the surfaceof the showerhead 20, which faces the semiconductor substrate 9,

[0056] da is the average distance between the showerhead 20 and thesusceptor 3 at an outer perimeter position 23 of the semiconductorsubstrate 9,

[0057] dc is the average distance between the showerhead 20 and thesusceptor 3 at a point on a radius of a distance equivalent to da fromthe center 22 of the semiconductor substrate 9. A deformation ratio fdaccording to this invention is fd=1%˜100%, preferably 5˜35%. Thedeformation ratio fd differs according to the type of reaction gassupplied, the mixing ratio, the RF power applied, and other factors, andthe most suitable value is selected.

[0058]FIG. 3 shows variation[example]s of the above-mentioned firstexample of this invention. In the first variation example shown in FIG.3(a), the base of the showerhead 20 comprises a rotating surface whosepart facing the semiconductor substrate is largely concave with itscenter 24 a protruding. In the second variation example shown in FIG.3(b), the base of the showerhead 20 b is concave in a substantiallyconical shape and its center 24 b protrudes. In the third variationexample shown in FIG. 3(c), the base of the showerhead 20 c has twoconcave parts and the center 24 c is nearly flat.

[0059] Thus, the structure of the showerhead 20 of this invention is notrestricted to the one shown in the first example, for which a distancebetween the showerhead 20 and the susceptor 3 becomes greatest at thecenter. In other words, the structure of the showerhead according tothis invention is substantially characterized in the regard that thepart facing the semiconductor substrate is concave, and for that concavestructure, the most suitable one is selected according to thespecifications of the showerhead and the susceptor, the RF power, andother film-forming conditions.

[0060]FIG. 4 roughly illustrates the second example of this invention.This works in the same as a conventional plasma CVD film-forming device,but in this second example, the surface 31 of the susceptor 30 comprisesa concave rotating surface. The showerhead 2 is the same flat-typeshowerhead and constitutes the upper electrode. The distance between thesusceptor 30, i.e., the lower electrode, and the showerhead 2 isgreatest at the central point 33 and gradually lessens toward the outeredge along a radius. The semiconductor substrate 9 contacts thesusceptor solely at its rim 32, and it can be fixed by, for example,vacuum fastening.

[0061] The deformation ratio fd′ of the central part of the lowerelectrode 30 is defined as follows:

fd′=|dc′−da′|/da′×100

[0062] wherein:

[0063] fd′ is the deformation ratio of the central part of the surfaceof the susceptor 30, which faces the semiconductor substrate 9,

[0064] da′ is the average distance between the showerhead 2 and thesusceptor 30 at the outer perimeter position 34 of the semiconductorsubstrate 9,

[0065] dc′ is the average distance between the showerhead 20 and thesusceptor 30 at a point on a radius of a distance equivalent to da′ partfrom the center 22 of the semiconductor substrate 9. The deformationratio fd′ according to this invention is fd′=1˜100%, preferably 5˜15%.The deformation ratio fd′ differs according to the type and mixing ratioof reaction gas, the RF power applied, and other factors, and the mostsuitable value is selected.

[0066] At this point it should be noted that a variation example similarto the variation example of the first example shown in FIG. 3 isapplicable to the susceptor 30 of this invention. In other words, thestructure of the susceptor 30 of this invention is not restricted tothose shown in the second example, for which the distance between theshowerhead and the susceptor becomes greatest at the central part.

[0067] Next, the third example of this invention is roughly illustratedin FIG. 5. This works in the same way as a conventional plasma CVDfilm-forming device, but in the third example, the respective surfaces21 and 31 of the showerhead 20 and the susceptor 30 comprise concaverotating surfaces. The showerhead 20 has the same rotating surface 21,which is concave at the center, as that of the first example and itconstitutes the upper electrode. Similarly to the second example, thesusceptor 30 comprises a rotating surface that is concave at the center.The distance between the susceptor 30 and the showerhead 20 is greatestbetween respective central points 33 and 24 and it gradually lessenstoward the outer edge along a radius. The semiconductor substrate 9contacts the susceptor only at its rim 32 and is fixed by, for example,vacuum fastening.

[0068] The deformation ratio fd for the third example according to thisinvention is fd=1˜100%, preferably 5˜35% (in another embodimnent,fd=10˜35%). The deformation ratio fd differs according to the type andmixing ratio of reaction gas, the RF power applied, and other factors,and the most suitable value is selected.

EXAMPLE

[0069] The experimental results of this invention are explained below.

[0070] The experiment intends to measure each film thicknessdistribution obtained using two types of showerheads according to thefirst example of this invention.

[0071]FIG. 6 is a graph showing the shape of the surface of eachshowerhead. Rotating surfaces on the showerhead base are formed byrotating the respective shapes a and b on the central axis of theelectrode used as a rotating axis. As a result, a difference results inthe electrode interval toward the radius.

[0072] The experiment was conducted under the following conditions:

[0073] Distance da between the electrodes at the outer rim of thesemiconductor substrate=10 mm

[0074] The depth of the concave surface at the center 24 of theshowerhead a (the degree of concavity)=1 mm, deformation ratio fd=11%

[0075] The depth of the concave surface at the center 24 of theshowerhead b (the degree of concavity)=3 mm, deformation ratio fd=32%

[0076] Ø of the semiconductor substrate used=200 mm

[0077] Temperature at the lower electrode=400° C.(752° F.)

[0078] Frequency f of RF power source used=13.56 MHz

[0079] Material gas=DM-DMOS, flow=20 sccm

[0080] Material gas=Ar, flow=10 sccm

[0081] Material gas=He, flow=10 sccm

[0082] From the experimental results shown in FIG. 7, while the filmthickness accumulated on the semiconductor substrate around the centralpart of the showerhead electrode was approximately 6% thicker for aconventional parallel-flat-plate type plasma CVD film-forming devicethan the average film thickness, the film thickness accumulated on thesemiconductor substrate around the central part of the showerheadaccording to this invention was improved to remain 1.5% thicker than theaverage film thickness and the film thickness of a thin film accumulatedon the semiconductor substrate around the central part of the showerheadb resulted conversely in a 2.5% thinner than [the] average filmthickness.

[0083] From these experimental results, it was found that the evennessof a film could be improved by forming electrodes so that the distancebetween the electrodes becomes greater around the central part of thesemiconductor substrate, thereby adjusting the plasma electric field toevenly distribute its intensity.

[0084] Alternatively, the direction of thermal expansion of theelectrodes when forming a film on the semiconductor substrate changes inthe direction of narrowing the electrode interval, or conversely, in thedirection of widening it according to a method of fixing the outerperimeter of electrodes, residual stress of an electrode surface, subtledeflection of a surface shape or a shape of a fine hole for supplyingreaction gas, etc.

[0085] Conventionally, it was difficult to control changes in thisdirection to be constant at all times. If the distance between theelectrodes is shortened, the electric field around the central part ofthe semiconductor substrate becomes very strong, the growth rate of thefilm also increases and the evenness of the film deteriorates.

[0086] However, according to this invention, by making the structure ofthe central part concave from the beginning, the evenness of the filmaround the semiconductor substrate center further improves because theelectrodes expand only in the direction of widening the electrodeinterval.

[0087] Effects of the Invention

[0088] With an embodiment of the plasma CVD film-forming deviceaccording to this invention, it has become possible to form a thin filmon a semiconductor substrate evenly. As a result, demand for more highlyintegrated and higher performance semiconductor elements can beaddressed.

[0089] Moreover, with an embodiment of the plasma CVD film-formingdevice according to this invention, demand for more even and stable filmthickness and film quality can be addressed.

[0090] Furthermore, an embodiment of the plasma CVD film-forming deviceaccording to this invention makes it possible to sufficiently address alarger diameter of future semiconductor substrates and to form a thinfilm evenly across a wide area.

[0091] It will be understood by those of skill in the art that numerousand various modifications can be made without departing from the spiritof the present invention. Therefore, it should be clearly understoodthat the forms of the present invention are illustrative only and arenot intended to limit the scope of the present invention.

What is claimed is:
 1. A plasma CVD film-forming device for forming athin film on a substrate, which comprises: a vacuum chamber; ashowerhead positioned within said vacuum chamber; and a susceptorpositioned substantially in parallel to and facing said showerheadwithin said vacuum chamber and on which said substrate is loaded,wherein the showerhead and the susceptor are used as electrodes and havesurfaces facing each other, at least one of which surfaces is concave.2. The plasma CVD film-forming device according to claim 1, wherein theconcave surface is a rotatably symmetrical surface around an axis of theshowerhead or the susceptor.
 3. The plasma CVD film-forming deviceaccording to claim 1, wherein a distance between said showerhead andsaid susceptor satisfies the following relation: fd=|dc−da|/da×100fd=1%˜100% wherein: fd is a deformation ratio of the central part ofsaid showerhead's surface that faces said substrate, da is the averagedistance between said showerhead and said susceptor at an outerperimeter position of said substrate, dc is the average distance betweensaid showerhead and said susceptor at a point on a radius of a distanceequivalent to da from the center of said substrate.
 4. The plasma CVDfilm-forming device according to claim 3, wherein a distance betweensaid showerhead and said susceptor additionally satisfies the followingrelation: fd′=|dc′−da′|/da′×100 fd′=1%˜100% wherein: fd′ is adeformation ratio of the central part of said susceptor's surface thatfaces said substrate, da′ is the average distance between saidshowerhead and said susceptor at an outer perimeter position of saidsubstrate, dc′ is the average distance between said showerhead and saidsusceptor at a point on a radius of a distance equivalent to da′ fromthe center of said substrate.
 5. The plasma CVD film-forming deviceaccording to claim 3, wherein deformation ratio fd is 5-35%.
 6. Theplasma CVD film-forming device according to claim 3, wherein deformationratio fd is determined to render substantially uniform a distribution ofelectric field intensity over the substrate while forming a filmthereon.
 7. The plasma CVD film-forming device according to claim 3,wherein distance da is in the range of 3 to 300 mm.
 8. The plasma CVDfilm-forming device according to claim 3, wherein distance dc is in therange of 3.3 to 350 mm.
 9. The plasma CVD film-forming device accordingto claim 1, wherein a distance between said showerhead and saidsusceptor satisfies the following relation: fd′=|dc′−da′|/da′×100fd′−1%˜100% wherein: fd′ is a deformation ratio of the central part ofsaid susceptor's surface that faces said substrate, da′ is the averagedistance between said showerhead and said susceptor at an outerperimeter position of said substrate, dc′ is the average distancebetween said showerhead and said susceptor at a point on a radius of adistance equivalent to da′ from the center of said substrate.
 10. Theplasma CVD film-forming device according to claim 9, wherein deformationratio fd′ is 5-35%.
 11. The plasma CVD film-forming device according toclaim 9, wherein deformation ratio fd′ is determined to rendersubstantially uniform a distribution of electric field intensity overthe substrate while forming a film thereon.
 12. The plasma CVDfilm-forming device according to claim 9, wherein distance da′ is in therange of 3 to 300 mm.
 13. The plasma CVD film-forming device accordingto claim 9, wherein distance dc′ is in the range of 3.3 to 350 mm. 14.The plasma CVD film-forming device according to claim 1, wherein theshowerhead supplies a material gas containing a compound selected fromthe group consisting of compounds which can be expressed bySi_(x)O_(y)C_(z)N₁H_(m), wherein x, y, z, 1, and m are independentlyzero or an integer, including SiH₄, Si(OC₂H₅)₄, (CH₃)₂Si(OCH₃)₂, andC₆H₆.
 15. The plasma CVD film-forming device according to claim 1,wherein a distance dw between the susceptor and the substrate is in therange of 0.1 to 10 mm.
 16. The plasma CVD film-forming device accordingto claim 1, wherein the susceptor has a diameter sufficient to support asubstrate having a diameter of 300 mm or larger.
 17. The plasma CVDfilm-forming device according to claim 1, wherein an radiofrequencypower is applied between the showerhead and the susceptor.
 18. Theplasma CVD film-forming device according to claim 1, wherein thesusceptor comprises a heater.
 19. A method for forming a thin film on asubstrate by using a plasma CVD film-forming device comprising: a vacuumchamber; a showerhead positioned within said vacuum chamber; and asusceptor positioned substantially in parallel to and facing saidshowerhead within said vacuum chamber and on which said substrate isloaded, wherein the showerhead and the susceptor are used as electrodesand have surfaces facing each other, at least one of which surfaces isconcave, said method comprising: loading a substrate on the susceptor;controlling the atmosphere in the vacuum chamber; applying energybetween the showerhead and the susceptor; and forming a thin film on thesubstrate.
 20. The method according to claim 19, wherein the concavesurface is a rotatably symmetrical surface around an axis of theshowerhead or the susceptor.
 21. The method according to claim 19,wherein a distance between said showerhead and said susceptor satisfiesthe following relation: fd=|dc−da|/da×100 fd=1%˜100% wherein: fd is adeformation ratio of the central part of said showerhead's surface thatfaces said substrate, da is the average distance between said showerheadand said susceptor at an outer perimeter position of said substrate, dcis the average distance between said showerhead and said susceptor at apoint on a radius of a distance equivalent to da from the center of saidsubstrate.
 22. The method according to claim 19, wherein a distancebetween said showerhead and said susceptor additionally satisfies thefollowing relation: fd′=|dc′−da′|/da′×100 fd′=1%˜100% wherein: fd′ is adeformation ratio of the central part of said susceptor's surface thatfaces said substrate, da′ is the average distance between saidshowerhead and said susceptor at an outer perimeter position of saidsubstrate, dc′ is the average distance between said showerhead and saidsusceptor at a point on a radius of a distance equivalent to da′ fromthe center of said substrate.